Directional drilling involves controlling the direction of a wellbore as it is being drilled. It is often necessary to adjust the direction of the wellbore frequently while directional drilling, either to accommodate a planned change in direction or to compensate for unintended and unwanted deflection of the wellbore.
In the drill string, the bottom-hole assembly is the lower portion of the drill string consisting of the bit, the bit sub, a drilling motor, drill collars, directional drilling equipment, and various measurement sensors. Typically, drilling stabilizers are incorporated in the drill string in directional drilling. The primary purpose of using stabilizers in the bottom-hole assembly is to stabilize the bottom-hole assembly and the drilling bit that is attached to the distal end of the bottom-hole assembly, so that it rotates properly on its axis. When a bottom-hole assembly is properly stabilized, the weight applied to the drilling bit can be optimized.
A secondary purpose of using stabilizers in the bottom-hole assembly is to assist in steering the drill string so that the direction of the wellbore can be controlled. For example, properly positioned stabilizers can assist in increasing or decreasing the deflection angle of the wellbore by supporting the drill string near the drilling bit or by not supporting the drill string near the drilling bit.
Drilling operators frequently have a need to open up tight restrictions in a borehole prior to running casing, liners, and packers in the hole. In addition, reamers may be used to remove ledges in the borehole profile. Reaming a borehole reduces the frequency of stuck pipe, helps in running wireline tools that may get stuck on ledges, and reduces the frequency of stick slip, which reduces the amount of vibration and the damage to the bottom hole assembly and the drilling bit.
In addition, reaming or opening a borehole reduces the annular fluid velocities to manage the equivalent circulating density (ECD) more effectively, an important factor in the drilling of a well.